The transition from uterine quiescence to contraction is vital to the health of a newborn and mother, but timing of this event often fails to occur properly; in the U.S., 12% of babies are born preterm, and 20% are delivered following artificial induction of labor. Thus, understanding the regulation of myometrial smooth muscle cell (MSMC) electrical activity and its effect on contraction is essential for both comprehending normal labor and treating dysfunctional labor. Maintenance of uterine quiescence requires an intricate balance between excitatory depolarizing stimuli that promote contractions and inhibitory repolarizing currents that suppress uterine contraction. One predominant channel in MSMCs, the large conductance Ca2+-activated K+ channel (KCa1.1) contributes to quiescence by eliciting a potent repolarizing current in response to excitatory signals, thereby dampening MSMC contraction. In spite of strong evidence supporting the notion that the KCa1.1 channel modulates uterine excitability, the basic mechanisms involved in its physiological regulation during pregnancy remain largely uncharacterized. The long-term goal of my research is to identify the ionic mechanisms that regulate the transition from quiescence to contraction during pregnancy. The objective of this proposal is to define the mechanisms by which the KCa1.1 channel is modulated during pregnancy to control myometrial excitability. Our central hypothesis is that this channel is dynamically modulated by both intrinsic properties and by its association with modulatory proteins. In support of this idea, our preliminary studies in human MSMCs indicate that KCa1.1 is regulated by alternative translation initiation, resulting in KCa1.1 isoforms that vary in their extracellular N- termini. These N-terminal variants differ in their regulation by accessory 1-subunits. We also have generated proteomics data demonstrating that other novel modulators, including the recently described family of - subunits and the protease inhibitor 2macroglobulin (A2M), selectively associate with myometrial KCa1.1 and potentially modify channel activity. The goals of this project are to: 1) define the spatial and temporal interactions between novel proteins that interact with KCa1.1 in non-laboring and laboring human myometrium, 2) identify intrinsic properties of KCa1.1 that alter its association with interacting partners; and 3) determine the mechanism of functional regulation of KCa1.1 in both myometrial cell lines and non-laboring and laboring human myometrium. The research proposed here will establish the molecular pathways that regulate KCa1.1 activity, providing a biological basis for therapies designed to modulate uterine excitability.